Comparison of soluble and plastidal polyphenol oxidase in mature apples

Polyphenol oxidase (PPO) is transported to plastid after translation. However, we often detect PPO activity in a soluble fraction of apple. Here we examined the location of PPO in retail apples. PPO activity was detected primarily in the plastidal fraction in most apples, however, it was sometimes detected more strongly in the soluble fraction than in the plastidal fraction. Western blotting analysis detected PPOs in both plastidal and soluble fractions as the same band. Some apples having high soluble PPO activity showed a band of decomposed PPO in addition to an intact band. Plastidal, soluble, and solubilized PPOs had similar optimal pH at about 4 and K-m values (about 100 mum). However, solubilized PPO showed the highest V-max and soluble PPO the least value. Sonic part of PPO seems to be solubilized, denatured and proteolyzed during maturation and storage.